1. Field of the Invention
The present invention relates to a fuel cell system.
Priority is claimed on Japanese Patent Application No. 2003-386570, filed Nov. 17, 2003, the content of which is incorporated herein by reference.
2. Description of Related Art
One type of fuel cell is a fuel cell installed in a fuel cell vehicle and the like, and such a fuel cell generates electrical power by a chemical reaction of reaction gas. This type of fuel cell, for example, includes a fuel cell in which a solid polymer electrolyte membrane is sandwiched between an anode and a cathode. A fuel gas (e.g., hydrogen gas) is supplied to the anode, and an oxidizer gas (e.g., air containing oxygen) is supplied to the cathode, and chemical energy involved in the reduction in these reaction gases is extracted as electric energy.
In such a fuel cell, water is generated at the cathode side during the power generation, and a part of the generated water permeates to the anode side through the solid polymer electrolyte membrane. In addition, some of nitrogen in the air supplied to the cathode is mixed with the hydrogen gas in the anode through the solid polymer electrolyte membrane, although the amount of the infiltrated nitrogen gas is very small. Impurities at the anode side, such as water or nitrogen and the like, may make power generation of the fuel cell unstable.
In particular, the concentration of the impurities at the anode side tends to gradually increase in a circulation-type fuel cell system in which unreacted hydrogen discharged from the fuel cell is collected, mixed with fresh hydrogen gas, and supplied to the fuel cell again in order to increase the usage of hydrogen.
In this type of fuel cell, the impurities in the hydrogen gas are discharged by periodically opening a discharge valve in order to discharge hydrogen gas containing impurities from a hydrogen circulation path (see Japanese Unexamined Patent Application, First Publication No. 2000-243417, for example).
As a system which controls discharges of hydrogen gas containing impurities from a hydrogen circulation path by periodically opening a discharge valve, a system is known which includes a plurality of determining units for determining whether or not to open or close a discharge valve in part of the control flow of the system. The system determines whether or not to open or close the discharge valve based on a combination of decisions made by these determining units.
A fuel cell system is also known having a plurality of discharge valves, and the system includes at least one determining unit for determining whether or not to open or close the respective discharge valves, and a decision whether or not to open or close the discharge valves is made based on decisions made by each of the determining units.
However, in a conventional system having a plurality of determining units, when a valve is being opened or soon after the valve is opened based on a decision made by one of the determining units, another determining unit makes a decision to open the same discharge valve and the discharge valve is opened. Such a successive opening of a discharge valve would result in excessive discharging of hydrogen gas together with impurities, and the efficiency of power generation would deteriorate because it appears that the hydrogen consumption increases. In addition, in a fuel cell having a dilution device which dilutes hydrogen gas discharged from a hydrogen circulation path with a cathode off-gas which is the gas discharged from the cathode, it is difficult to control the concentration of the diluted hydrogen.
Furthermore, in a system including a plurality of determining units and a plurality of discharge valves, when a first discharge valve of the discharge valves is being opened or soon after the first discharge valve is opened based on a decision made by one of the determining units, another determining unit would make a decision to open any of the discharge valves, and according to the decision, opens a second discharge valve at the same time the first discharge valve is being opened or soon after the first discharge valve is opened. Such a successive or simultaneous opening of multiple discharge valves would cause excessive discharging of hydrogen gas together with impurities, and the efficiency of power generation deteriorates because it appears that the hydrogen consumption increases. In addition, in a fuel cell having a dilution device which dilutes hydrogen gas discharged from a hydrogen circulation path with a cathode off-gas which is the gas discharged from the cathode, it is difficult to control the concentration of the diluted hydrogen.